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Is the irrigation system on a hill such that once the system is turned off, the water in the line drains out of an emitter at the low end, sucking in air (and possibly some mulch) from the high end?

The other possibility is if your hydrant is lower than the emitter, the same thing could be happening when you disconnect the 1/2" line, or the hydrant might have a PVB (Pressure Vacume Breaker) that is working sort of like an anti-siphon valve. But in that situation, the PVB is just there to insure water doesn't flow back into the hydrant. If there is a hose connected to the hydrant, and the water in the hose is higher than the hydrant (causing the water to TRY to flow back into the hydrant, a PVB would protect the hydrant by discharging the water. Basically, the way the PVB works is that when there is water pressure from behind (i.e. the hydrant), it pushes a stop forward that covers drain hole and all the water goes in the hose. But when the water is shut off (i.e. no more pressure behind it) the excess pressure on the front side opens the drain holes and the water drains out. Now if that is happening, then the emitters should be sucking in air (and mulch) as the water drains.

By the way, do you have a filter on the water supply. Even if you have city water, there is still plenty of "stuff" that could clog drip irrigation. That is why ALL drip irrigation needs a FINE filter (150 mesh). If you go to one of the big box retailer and purchase a kit made to connect to a hose spicot, the kit will contain a backflow preventer (basically an extreamly simple form of PVB), a filter, and some connection to the 1/2" tubing usually used for drip irrigation.

It would depend on what equipment you're using. Do you have a brand name you can google and look for information with? Actual 'sprinkler heads' would be above grade. Even drip emitters are meant to be above grade. They make little tube supports for positioning, and even 'bug caps' to keep critters out of the tubing.

And you could have an antisyphon valve feeding your drip. It just takes some plumbing skills to connect one.

Can you shed any light on the problem I'm having with the bark mulch jamming up the Xeri-sprinkler heads? Do you think that's a result of a lack of an anti-siphon apparatus? But if so, how do you think the bark mulch finds its way into the system? The Xeri-sprinkler heads aren't down in the bark mulch--only the tubing, so what gives? Thanks for any additional info you can provide!

Gravity. AS valves, and vacuum breakers, work on gravity. Higher than anything they feed, with no shutoff valves downstream, and open to the atmosphere when there is no pressure in the device, the downstream water would have to defy gravity, and climb uphill in an open pipe, in order to cause contamination.

Clogged Xeri-sprayer Heads--Every Cycle

I installed a small drip system using 1/2" to 1/4" tubing, burying it under bark mulch to hide the tubing. The problem is that every time I turn the "system" on, the spray heads are filled with bark mulch pieces! I've checked for leaks and there aren't any. When I want to use that drip system, I just attach the 1/2" to the hydrant. There is no type of anti-siphon aparatus and I'm wondering if it's possible for those bits of bark mulch to get sucked in through any of the 1/2" or 1/4" connections. The connections all appear to be pretty tight, but I'm such an amateur--what do I know!? How can I prevent this problem in future installations?

Also, could any of you knowledgeable sprinkler gurus out there explain exactly how an anti-siphon valve works--not just what it does--but how it <i>actually works</i>? I've wondered about that and thought it's about time I got up to speed on some of these "mysteries."